Render very slow using GPU from Radeon Sapphire RX 580 vs CPU
First of all the GPU needs to compile the Kernel first ( which takes more time in case of your cube than the actual render itself ) - this is done on the cpu.

Second Radeon need bigger tiles to have enough work to even get remotely saturated. 256x256 is a bare minimum and in case 2.79 or below is used you should even go Fullscreen ( 1 or 2 tile ) for best performance ( dont use progressive ! ) - each call to the Gpu halts it for a short time - the more of theese calls for example when progressive is used come together - like when eavery sample needs to be updated to the cpu the gpu spends most of the time waiting for data.

Take a more complex scene and try again.